All Christian denominations agree with the Jewish ones: God rested on the seventh day - The Sabbath, or Saturday. Jesus was then resurrected on the day after the Sabbath, the first day of the Jewish week, which became the Lord's Day in many European languages (Domenica, Domingo, Duminică etc). It was quickly understood as a non-working day, a day of religious celebration, but it was still the first day of the week.

Now, since Jesus and his resurrection are much more important in Christianity than the creation of the world, the Sabbath began losing its importance, and so renumbering the week started making sense, but was a contentious religious and political issue for a long time. Of course, having two rest days, and one of them being based on Jewish customs in a quite anti-semitic medieval Europe, played a part as well in wanting to change things.

I'm relatively sure that the Portuguese week day names are based on the original Jewish week.